This invention relates to a semiconductor integrated circuit device having an MCU (micro controller unit) and more particularly to the technique for reducing the possibility of the erroneous operation of an MCU due to introduction of electromagnetic disturbance noise to a reset terminal, test terminal, mode switching terminal, interruption terminal and the like.
A reset circuit for initializing the operation of an MCU is contained in the semiconductor integrated circuit device having the MCU and a reset terminal for inputting a reset signal used for specifying the initialization is formed on the reset circuit. Further, a test terminal for inputting a test signal to effect the test operation, a mode switching terminal for inputting a mode switching signal to switch the operation mode, and an interruption terminal for inputting an interruption signal to effect the interruption process and the like are provided.
However, a noise from the exterior, for example, noise from an actuator is introduced into the reset terminal, test terminal, mode switching terminal, interruption terminal and the like so as to cause the erroneous operation and runaway of the MCU. As a countermeasure for noises in the semiconductor integrated circuit device having the MCU, conventionally, a noise filter circuit as shown in FIG. 1 is provided between the reset terminal and the reset circuit, for example, to eliminate the noise.
The noise filter circuit shown in FIG. 1 includes an inverter IB100, delay circuit DL100 and NAND gate ND100. A reset signal RESET input to the reset terminal RES is supplied to and inverted by the inverter IB100, then the output signal of the inverter IB100 is input to the first input terminal of the NAND gate ND100 and input to the second input terminal of the NAND gate ND100 after it is delayed by the delay circuit DL100. Then, the reset signal RESET is output from the NAND gate ND100 and supplied to the reset circuit for initializing the MCU.
The noise filter circuit eliminates an impulsive noise shorter than the delay time of the delay circuit DL100 among the noises introduced into the reset terminal RES so as to prevent the noise from being introduced into the reset circuit.
However, in the noise filter circuit with the construction shown in FIG. 1, only an impulsive noise having a pulse width shorter than the delay time of the delay circuit DL100 can be eliminated. For this reason, a noise having a pulse width longer than the delay time will pass through the circuit and is input to the next-stage reset circuit. Therefore, if the noise is longer than the delay time of the delay circuit DL100 and shorter than time which is sufficiently long for causing the reset circuit to correctly function, it becomes impossible to initialize the MCU and the noise may cause the erroneous operation and runaway of the MPU.
The problem of the erroneous operation and runaway of the MCU due to the electromagnetic disturbance noise may occur not only in the reset terminal but also in the other terminal such as the test terminal, mode switching terminal and interruption terminal.